1. Field of the Invention
The present invention relates to an antenna device and a portable wireless communication apparatus, and more particularly, is suitably applicable, for example, to a portable wireless communication apparatus which is configured to correspond to at least two kinds of radio communication systems using different radio communication frequencies.
2. Description of the Related Art
As portable wireless communication apparatuses have rapidly prevailed in recent years, only a single radio communication system tends to be incapable of providing a sufficient number of circuits. It is therefore conceived to reserve a necessary number of circuits by using another radio communication system which uses a different frequency bands and, owing to remarkable progresses made in a technology for compact and light-weight configurations, there has been developed a terminal which allows a single portable wireless communication apparatus to use two kinds of radio communication systems.
On the other hand, an amount of electromagnetic waves to be absorbed by specific regions of a human body (mainly a head) per unit time and unit mass out of electromagnetic waves emitted from a portable wireless communication apparatus is defined as an average local Specific Absorption Rate (SAR) of the portable wireless communication apparatus and it is demanded to restrict a maximum value of this SAR to a specified value or lower.
In FIG. 1, reference numeral 1 denotes a portable wireless communication apparatus which is developed so as to suppress a maximum value of the local average SAR to a specified value or lower as a whole. In the Figure, a circuit substrate (not shown) required for radio communication is accommodated in a cabinet (not shown) made of a non-conductive material and covered with a shield case 2 used as a ground member.
Since the internally accommodated circuit substrate is covered with the shield case 2, this portable wireless communication apparatus 1 prevents a transmitting-receiving circuit and other various kinds of circuits mounted on the circuit substrate from producing adverse influences on one another, an antenna 4 and other appliances.
Furthermore, the internal circuit substrate is configured to generate a transmitting-receiving signal of a predetermined format with the transmitting-receiving circuit for communication with a base station, transmit the signal from the antenna 4 to the base station by way of an antenna power supply portion 3, and demodulate a reception signal which is received with the antenna 4 and accepted by way of the antenna power supply portion 3.
The antenna 4 is, for example, a bar like rod antenna which is made of a conductive wire material, but the portable wireless communication apparatus is configured to be capable of using other various types of antennas such as a helical antenna which is made of a conductive wire material wound in a spiral form and an expansion type antenna which is a composite type of the rod antenna and the helical antenna.
Only the above described antenna 4 does not function as an antenna, but a high-frequency current is supplied also into a ground conductor of the circuit substrate or the shield case 2, whereby the portable wireless communication apparatus 1 as a whole functions as an antenna.
The portable wireless communication apparatus 1 is configured to measure the local average SAR during communication and it has been confirmed that a spot at which the local average SAR has a maximum value (hereinafter referred to as a hot spot) is in the vicinity of an ear which is in contact with a speaker 7 as shown in FIG. 2.
A reason is considered that the portable wireless communication apparatus 1 is used in a condition where the speaker 7 is kept in contact with an ear of a human body during communication and the ground conductor of the circuit substrate existing on a rear side of the speaker 7 or the shield case 2 which functions as a portion of the antenna emits electromagnetic waves.
The portable wireless communication apparatus 1 (FIG. 1) therefore has a conductive planar plate 5 disposed at a location which is opposed to the speaker 7 (not shown) and slightly floated from a top surface 2A of the shield case 2 so as to be nearly in parallel with the top surface 2A.
By the way, a gap between the conductive planar plate 5 and the top surface 2A of the shield case 2 is determined dependently on radio communication frequencies and the portable wireless communication apparatus 1 is configured to be capable of adjusting a frequency bandwidth dependently on the above described gap.
An end of the conductive planar plate 5 is short-circuited to the shield case 2 by a short-circuiting conductor 6, the other end of the conductive planar plate 5 is electrically open from the shield case 2 upward in a direction indicated by an arrow a and a distance L1 from the short-circuited end to the open end is selected so as to be a wavelength xcex at a radio frequency/4.
Accordingly, impedance between the conductive planar plate 5 and the shield case 2 of the portable wireless communication apparatus 1 is nearly xe2x80x9c0xe2x80x9d at the short-circuited end but close to infinity at the open end, whereby the high-frequency current is hardly supplied from the vicinity of the antenna power supply portion 3 to the conductive planar plate 5 and the shield case 2.
By the way, it has experimentally proved that input impedance is 0 at the short-circuited end and input impedance is a maximum at the open end when the distance L1 as measured from the short-circuited end to the open end of the conductive planar plate 5 is selected as the wavelength xcex at the radio communication frequency/4, and that input impedance is 0 at the open end when the distance L1 as measured from the short-circuited end to the open end is selected as the wavelength xcex at the radio communication frequency/2.
Accordingly, the portable wireless communication apparatus 1 makes the high-frequency current hardly supplied to the conductive planar plate 5 and the shield case 2, thereby being capable of reducing an amount of electromagnetic waves emitted from the conductive planar plate 5 and the shield case 2, and lowering the local average SAR in the vicinity of the ear.
In the portable wireless communication apparatus 1 having the configuration described above, however, the distance L1 from the short-circuited end to the open end of the conductive planar plate 1 is determined by a radio communication frequency to be used, and even when the distance L1 from the short-circuited end to the open end of the conductive planar plate 5 is a wavelength xcex/4 and impedance is maximum at the open end at a radio communication frequency of 900 MHz, for example, the length L1 from the short-circuited end to the open end of the conductive planar plate 5 corresponds to a wavelength xcex/2 at a radio communication frequency of 1.8 GHz.
Accordingly, the portable wireless communication apparatus 1 allows impedance to be lowered at the open end of the conductive planar plate 5 and increases an amount of electromagnetic waves emitted from the conductive planar plate 5 and the shield case 2, thereby being incapable of lowering the local average SAR at the radio communication frequency of 1.8 GHz though the portable wireless communication apparatus 1 allows impedance to be maximum at the open end of the conductive planar plate 5 and reduces an emitted amount of the electromagnetic waves, thereby being capable of lowering the local average SAR in the vicinity of the ear at the radio communication frequency of 900 MHz.
Accordingly, it is difficult for the portable wireless communication apparatus 1 to lower the local average SAR with the conductive planar plate 5 in correspondence to two kinds of radio communication systems which use different radio communication frequencies.
In view of the foregoing, an object of this invention is to provide an antenna device and a portable wireless communication apparatus which are compact, simple in configurations and capable of reducing an amount of electromagnetic waves to be absorbed by a human body in correspondence to at least two or more kinds of radio communication systems which use different radio communication frequencies respectively even when any radio communication frequency is used.
The foregoing object and other objects of the invention have been achieved by the provision of an antenna device and a portable wireless communication apparatus. The antenna device functions as an antenna by supplying electric power to an antenna element from a power supply point and supplying high-frequency currents to grounding conductors from the power supply point, and comprises high-frequency current restricting means which comprises at least: a first conductive planar plate having a first short-circuit portion where one end is electrically short-circuited to the grounding conductors, and a first open end portion where the other end is electrically opened and is positioned to bring input impedance close to infinity at first radio communication frequencies; and a second conductive planar plate having a second short-circuit portion where one end is electrically short-circuited to the grounding conductors, and a second open end portion where the other end is electrically opened and is positioned to bring input impedance close to infinity at second radio communication frequencies, and the first conductive planar plate and the second conductive planar plate are composed as one unit.
Since the input impedance at the open ends of the conductive planar plates can be brought close to infinity at the plurality of radio communication frequencies respectively, it is possible to limit radiation of electromagnetic waves by restricting the high-frequency currents supplied to the above described conductive planar plates and grounding conductors, thereby securely reducing an amount of electromagnetic waves to be absorbed by a human body in correspondence to at least two or more radio communication systems which use different radio communication frequencies even when any radio communication frequency is used.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.